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Difference Between a Seismic Wave & a Tsunami

Violent, natural, environmental events underneath Earth's crust, underwater or above Earth's surface -- earthquakes, volcanoes, explosions -- can set off a series of shock waves that radiate outward from the source of the event. The shock waves may appear as either seismic waves or tsunamis, which are capable of leaving a devastating amount of damage in their wake.

Seismic Waves and Tsunamis Defined
- Seismic waves are earth vibrations that earthquakes or explosions produce. The resulting shock wave causes an elastic change -- temporary deformation -- in the rocky earth structures as the waves pass by. Tsunamis are large sea waves, up to 30 meters, or 98 feet, in height, that can travel at speeds of up to 589 mph. Seaquakes, which are submarine earthquakes, and undersea volcanic eruptions cause tsunamis.
Seismic Waves
- Seismograms, tracings illustrating the intensity of seismic waves, are recorded on a seismograph. Seismic waves move quickly -- kilometers per second (km/s) -- depending on various factors, most importantly, rock composition. When an earthquake strikes, the seismic waves move outward from the epicenter, like waves in a pond after a rock is tossed in it. The earthquake sends out a ring of P-waves (primary, strong waves) followed by a ring of S-waves (secondary, shallow waves). By measuring the time between the P-waves and the S-waves, the seismic wave speed can be determined. The travel time of the seismic waves can also be determined by finding the distance between the earthquake and the seismometer, then dividing that distance by the seismic wave speed.
Tsunamis
- Tsunamis are seismic sea waves that were once incorrectly called tidal waves, since they have nothing to do with the tide. Underwater disturbances generate tsunamis, which can move at speeds of 100s of miles per hour on open water, until they reach shallower waters, producing waves as high as 100 feet or greater. In deeper waters, the wave has plenty of room to move in its circular motion, and the wave action may appear so shallow that it can't be felt by even the smallest fishing boats. The tsunami may consist of more than one wave, and succeeding waves may be stronger than the first. Areas at greatest risk from tsunamis are those within one mile of the shore and less than 25 feet above sea level.
Diffences Between Tsunamis and Seismic Waves
- Seismic waves, waves of energy produced by the sudden breaking of rocks, create body waves (waves that travel deep, through the earth) and surface waves (shallow waves traveling just under the Earth's crust). Tsunamis are waves with very long wavelengths between the peaks generated in a body of water.

Nature